The international telecommunication needs for data transmission are rapidly growing in recent years, the main drivers being video, supercomputing applications and data traffic via mobile communications. Under these conditions, terrestrial optical fiber transmission systems, such as DWDM (Dense Wavelength Division Multiplexed) transmission systems have been developed to meet the increasing demands for more cost-effective, bandwidth-rich telecommunication service. Furthermore, in order to accommodate the different types of traffic on the network (e.g. voice, data, video) and to increase the utilization of the available bandwidth in the network, network architectures migrate from circuit switched networks (which reserve bandwidth whether used or not), to packet switched networks.
Such migration implies a shift away from the synchronized network architecture, like SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy). SONET and SDH were originally designed to transport circuit mode communications, e.g. DS1, DS3, from a variety of different sources. Additionally, these network architectures support real-time, uncompressed, circuit-switched voice encoded communications in a so-called PCM (Pulse Code Modulation) format. Circuits in SONET/SDH are provisioned with a fixed reserved capacity in a so-called VC (Virtual Container) inside the SONET/SDH payload. Furthermore, a network architecture such as SDH uses a basic transmission format, i.e. an STM-1 (Synchronous Transport Module, level 1) frame. The distribution of timing, and thus network synchronization, is natural. In the network nodes, a reference frequency is retrieved from a received signal via a clock recovery circuit. Only the position of the VC relative to the position of the STM-1 frame needs to be known, which results in an absence of the need of an absolute timing reference for proper circuit operation.
Packet switched networks do not use such a fixed framing architecture, and therefore do not have a natural synchronized timing distribution. In these networks data frames need to be transmitted without overlap with other frames and with as little as possible unused and stranded capacity. Therefore, packet switched networks have stringent requirements concerning frequency dissemination as well as phase and time synchronisation to guarantee proper call connection and data transport, such as, for example, transfer of streamed data. These communication networks thus do need an absolute timing reference.
As the vast majority of mobile backhaul networks are upgraded to (Carrier) Ethernet, the base stations become unsynchronized. The use of GPS (Global Positioning System) receivers for providing an absolute timing reference, such as time of day information, in combination with a local oscillator for providing a reference frequency could be a solution. However, this approach is expensive and can easily be tampered with. Currently, the two most widely used methods to synchronize the base stations are 1) the ITU-T Synchronous Ethernet (G.8261) protocol and 2) the so-called Precision Time Protocol (PTP) described in IEEE-Standard 1588 version2.
The Synchronous Ethernet solution is centered on very similar frequency dissemination as used for SONET/SDH networks. That is the frequency in each network node is derived from the frequency from upstream nodes. In terms of frequency, the Synchronous Ethernet solution provides excellent frequency synchronization of the nodes in the network between base stations. However, Synchronous Ethernet is not capable of providing accurate time and phase information as is needed in packet networks. For example, Synchronous Ethernet does not take effects such as latency into account.
PTP is also similar to the SONET NTP (Network Time Protocol). PTP is a 2-way time transfer protocol with hardware time stamping. The master and slave end-points transport the timing information within MAC (Media Access Control) frames. Using PTP avoids the need to upgrade the intermediate nodes. Furthermore, PTP enables time and day synchronization, which is required for Time Division Duplex (TDD) mobile networks. However, to use PTP, additional hardware and boundary clocks are needed for regeneration and jitter compensation of timing signals in extensive networks. Furthermore, with PTP only limited synchronization accuracy can be achieved, typically of the order of 1 microsecond.